To abate the NOx content in the exhaust gas of lean-burn gasoline engines and in particular diesel engines, designated NOx after-treatment systems are required. In three-way catalysis the reduction of NOx to N2 is possible under nearly stochiometric conditions. Under the oxidizing conditions of a lean-burn engine this reduction is not possible. Therefore, a special exhaust gas after-treatment catalyst has been developed containing a material that is able to store NOx, e.g. as a nitrate under lean conditions. By applying short stoichiometric or rich operation conditions the stored NOx can then be converted to nitrogen and the storage material regenerated. This catalyst is commonly called a (lean) NOx-trap catalyst and is described for example in EP 1317953 A1.
As stated in EP 1317953 A1 a NOx-trap catalyst contains a nitrogen oxides storage material consisting of storage components, deposited on suitable support materials in a highly dispersed manner in order to create a large area of interaction with the exhaust gas. Materials that are able to store nitrogen oxides in the form of nitrates are for example the oxides, carbonates or hydroxides of alkaline earth metals, particularly of Barium.
A suitable support material has to provide a high specific surface area and a high thermal stability to ensure long-term durability of the final catalyst. Furthermore, the chemical composition and properties of the support material influences the nitrogen oxide conversion efficiency and the temperature operation window of the NOx-trap catalyst.
A beneficial material regarding the before mentioned properties that is well described in the art for use as a support material is a homogenous Mg/Al mixed oxide, with a magnesium oxide concentration of 1 to 40 wt. % based on the total weight of the oxide.
The term Mg/Al mixed oxide describes a mixture of the two oxides on an atomic scale and therefore excludes a physical mixture of the two individual oxides as well as materials prepared by impregnation of aluminium oxide with a magnesium oxide precursor solution. This type of Mg/Al mixed oxide is preferably obtained by calcination of a Mg/Al mixed oxide precursor that is obtained via hydrolysis of a mixture of the alkoxides that is for example described in detail in DE 19503522 A1 (=U.S. Pat. No. 6,517,795).
Further improvements of the activity and durability of a NOx-trap catalyst were achieved by using the homogenous Mg/Al mixed oxide support materials that further have been doped with a cerium based oxide, for example by coating the Mg/Al mixed oxide with cerium oxide and/or praseodymium oxide. WO 2005/092481 A1 claims the observed benefits in the catalyst performance to be closely associated with the doping of the Mg/Al mixed oxide with cerium oxide when compared to a physical mixture of Mg/Al mixed oxide with cerium oxide.
However, it turned out that when a cerium oxide doped homogenous Mg/Al mixed oxide used as a state-of-the-art support material is combined with a suitable barium compound as nitrogen oxide storage component, a thermal aging treatment (at 850° C. for 4 h) leads to the formation of BaAl2O4. It is well known (e.g. Jang et. al, Catalysis Letters 77 (2001) 21) that the formation of BaAl2O4 by reaction of the storage component with parts of the support material leads to deterioration of the NOx trap efficiency of the storage material because this compound has no ability to store nitrogen oxides in the form of nitrates. Therefore, in order to further enhance the thermal durability of NOx trap catalysts there is a need for support materials having an improved stability against the formation of inactive BaAl2O4.
Materials comprising manganese oxide and/or a mixture of cerium and manganese oxide for use in automotive emission control catalysts are known in the art. For example US 20120240554 A1 claims the use of MnOx—CeO2 mixed oxide particles as oxidation catalysts to promote the conversion of NO to NO2.
The use of oxides of manganese with cerium in conjunction with aluminium oxide in NOx-trap catalysts is known in the art and is taught by Le Phuc (Le Phuc, et al: “Nox Removal Efficiency and Ammonia Selectivity during the NOx storage-reduction process over Pt/BaO (Fe,Mn,Ce)Al2O3 model catalysts. Part II—Influence of Ce and Mn—Ce addition, Applied Catalysis B; Environmental 362-371). In this reference nitrate salts of Mn and Ce were simultaneously added with barium nitrate to alumina by an impregnation process. This document discloses that the addition of Mn and Ce improve storage rates and that a beneficial effect is observed when the Ce and Mn are added together. The problem of BaAl2O4 is not discussed by the prior art.
Further, as will be shown in the body of the specification, the prior art materials when undergoing a thermal aging treatment (at 850° C. for 4 h) lead to the formation of BaAl2O4, the very same problem that the present invention addresses.
The object of the present invention is therefore to provide a support material applicable in NOx trap catalysts that comprises a homogenous Mg/Al mixed oxide that is preferably doped with a cerium based oxide to offer the advantageous properties described above and additionally is stable against the reaction with the barium storage component to form BaAl2O4.